Anthropologists have been studying y-chromosomal dna to understand where and when humans first appeared and when they moved into various areas of the world. The y chromosome is found only in males, and most of the dna found on the y chromosome is passed directly from fathers to sons. Researchers collect y-chromosomal dna information from living men and measure the amount of genetic difference among populations. Greater amounts of genetic difference indicate longer periods of separation between different groups of people because it takes time to accumulate genetic variations. This y-chromosomal dna research has helped anthropologists to understand our species' origin in africa and our various migrations to other parts of the world.